In fiber-reinforced metal (hereinafter referred to briefly as FRM), physical characteristics not found in the metal itself have been realized by the combination of the metal with a fiber. Representative species of the hitherto-known FRM include aluminum alloys reinforced with carbon fiber, ceramic fibers [SiC fiber, Si-Ti-C-O fiber (tilano fiber), alumina fiber, boron fiber, etc.] or glass fiber, in which high tensile strength and rigidity and low coefficients of thermal expansion have been imparted to matrix metals.
However, in the conventional FRM, where the reinforcing fiber is carbon fiber, for instance, a reaction takes place between molten aluminum and reinforcing fiber to form aluminum carbide Al.sub.4 C.sub.3 so that the strength of FRM is not much increased. SiC or Si-Ti-C-O (tilano fiber), which is a ceramic fiber, cannot give a FRM with sufficient strength because of poor wetting by aluminum. Moreover, fiber containing SiO.sub.2, which is among alumina fibers, reacts with molten aluminum, while SiO.sub.2 -free alumina fiber is not reactive and good in wetting property but is low in strength (141 kg/mm.sup.2) . Moreover, boron fiber is so large in fiber diameter that it does not lend itself well to complicated product configurations so that it connot function well as a reinforcing fiber for FRM. These ceramic fibers have the additional serious disadvantage of high costs. Kevlar, which is a high-strength organic fiber, is low in heat resistance, and glass fibers in general are reactive with molten alloys and low in elasticity and strength.
Thus, the conventional FRM, thus made up of reinforcing fiber and matrix metal, are disadvantageous for example in that the reinforcing fiber and matrix metal react at their interfaces of the fiber can hardly be wetted by the molten metal. Thus, if the metal matrix reacts with the reinforcing fiber across the interface in the course of manufacture, the strength of the product FRM is adversely affected. On the other hand, if the wettability of the reinforcing fiber by the matrix metal is poor, there is obtained no sufficient bond between the two components so that the resulting FRM cannot enjoy the benefit of combination in strength and elasticity, thus failing to exhibit satisfactory physical characteristics.
For this reason, it has been proposed and practiced, in the manufacture of FRM, to plate the surface of the reinforcing fiber with nickel or other metal or treat the surface with an inorganic compound, such as SiC, by the plasma coating technique, before impregnation of matrix metal. However, such processes are costly and involve complicated procedures.
In view of the above-mentioned disadvantages, the inventors of the present invention explored the possibility of using oxynitride glass fiber, which is a high-elasticity fiber, as the reinforcing fiber for FRM and found that an oxynitride glass containing at least a certain proportion of nitrogen is not reactive with molten metal even in the absence of plating or other pretreatment and is highly wettable so that substantially the additive effect of the two components can be realized in the resulting FRM.